Prior flow meters have limitations of only sampling the volumetric flow or creating excessive head loss of the flow which causes a pressure drop as a fluid passes through the flow meter.
Traditional flow meters either rely on a tight frictional seal between the moving parts which causes greater head loss as the fluid passes therethrough or the moving parts have a loose seal and hence a greater amount of fluid is not accounted for by the positive displacement prior art chambers.
The present invention provides an increase in the amount of accuracy of measured fluid and requires very little pressure differential to rotate the rotor assembly. The present invention flow meter is a low friction, low fluid turbulence, positive displacement device with relatively linear operating characteristics over a wide range of operating speeds and operating fluid viscosities. This linearity will enable accurate and predictable flow metering of a wide range of operating fluids and flow rates with a simple rotational pulse-counter (as is common to the flow meter industry). There are many applications where pulse-counter positive displacement flow meters are used and where a wider flow range, small size, and increased accuracy of a pulse-counter flow meter would offer significant advantages.
Many other applications exist where even higher accuracy would be a significant advantage. Electronic flow meter control systems are common to the flow meter industry. In a second embodiment a control system which works on a unique set of inputs to refine the accuracy of the mechanical components. The control system applied to the rotor assembly has the potential to achieve higher accuracy then is currently available.
The rotors of the present invention and casing will offer excellent repeatability due to low friction and the positive displacement characteristic of the rotors. An obstacle to accurately measure fluid flow is to account for the changing parameters of viscosity, flow and system pressure accurately enough to predict how the rotors will perform under various conditions.
The three main characteristics which will affect the CvR™ flow meter will be fluid viscosity, flow rate, and overall system pressure.
The low flow resistance of the present invention allows higher viscosity fluids to be metered without damage to the flow meter and without causing unacceptable changes in the flow meter operating performance.
The present invention has three distinct advantages which benefit the accuracy and flow range capability of the device. The first characteristic is low mechanical friction. The CvR™ rotors do not contact during operation due to a fluid film between the rotors, and are only limited by the friction of the bearings or bushings. Even with oversized preloaded bearings, the pressure differential required to rotate the flow meter of the present invention rotors has been found to be only 0.5 psi. With low friction bearings, the pressure required to rotate the rotors can be less than 0.05 psi. Low mechanical friction allows the flow meter of the present invention to begin measuring flow with very low pressure differentials providing high accuracy at low flow rates with very little seepage of fluid past the rotors. The lower the pressure differential on either side of the rotors, the less seepage will occur past the rotor seals and because the rotors have so little resistance to rotation, a very low pressure differential will cause the rotors to turn. In other words, the rotors prefer to spin rather then lose fluid (and accuracy) past the seals.
The second characteristic of the flow meter of the present invention which is related closely to the flow rate parameter is low flow resistance that results from the low turbulence characteristic of the rotors that allows very high rotational speeds without excessively high pressure differentials across the rotors. This allows for greater flow rates through a smaller flow meter, and also increases the flow rate range so fewer models need to be produced. In addition to accuracy, increased flow range is very desirable as it will greatly simplify the job of an engineer who needs to account for the performance parameters of each flow meter in their fluid control system.
The third characteristic of the present invention is the lower the inertia of the rotors which results in a lower pressure differential required to cause them to accelerate during sudden changes in flow rate. The rotor assembly has only two moving parts and can be manufactured with a very low moment of inertia.
System pressure affects both the inlet and the discharge sides of the flow meter and therefore has very little effect on the rotors. The primary effect of system pressure is on the casing as it causes it to deform and increase seal gap clearances causing increased seepage and reduced accuracy. The high volume throughput of the flow meter of the present invention allows the use of a smaller volume than with other flow meter devices. This allows easier construction of a less deformable casing structure. In addition, it may be possible to construct the casing so the deformation which occurs with increased pressure actually decreases certain clearances making up for other areas which may increase.
All of the above characteristics are advantages of the flow meter of the present invention which can be further refined with electronic optimization. In a second embodiment the controller uses only three electronic inputs to monitor the changing parameters of fluid viscosity, flow rate, and system pressure and then to compare these values to a stored value database to determine the actual flow rate of fluid through the device at all times. The controller could also be designed to account for the affect of rotor inertia during sudden flow rate changes. Further the controller could account for the effect of temperature on the system with temperature sensors and calibrating the thermal expansion coefficients of the materials of the flow meter of the present invention.
As fluid viscosity of the operating fluid increases, the pressure differential across the flow meter of the present invention will increase due to increased flow resistance and there will be an increased tendency of the fluid to seep past the close tolerance seals. At the same time, however, increased viscosity also decreases the tendency of the fluid to seep past the seals and the two factors have opposing effects upon the flow.
In a preferred form, pressure transducers will be located in a stagnant flow area of the inlet and outlet ports. The viscosity of the fluid being metered will be determined by comparing the speed of the rotors determined with a simple pulse counter with the pressure differential which occurs at this speed. The higher the viscosity of the fluid at a given speed, the higher the pressure differential. A speed to pressure differential graph or function (based on empirical test data) will be stored in the controller and used to determine the viscosity of the fluid (at a predetermined sample rate).
Once the controller has determined the viscosity of the fluid, it will compare this viscosity value to another graph (or mathematical function) which will determine the appropriate flow rate correction at that rotor speed (based on empirical test data with a wide variety of viscosities). The two pressure transducers will also be used to determine the overall system pressure. This value will be compared to another graph (based on empirical test data) which will account for the effect of casing deformation on the flow measurement at that rotor speed and viscosity.
A special “micro-flow” condition will also be accounted for by the controller. This condition will be specified any time a micro-pressure differential is detected across the rotors but the rotors are not spinning. In this case, a very low volume of fluid will be seeping across the rotor seals. Empirical testing data is used to approximate this flow using the last recorded viscosity value for the fluid.